But, according to researchers at the State University of New York, part of the solution could come from an entirely different kind of plant—trees. Scientists have long known that trees can help us to reduce energy use and clean the air of pollutants and gases we emit into the atmosphere.
But not all trees are created equal.
As part of a research project, students and faculty members at SUNY's College of Environmental Science and Forestry have determined the precise combination of trees that would be most effective in reducing the level of greenhouse gases in the air around Syracuse, NY.
"It's one more step you could take to help combat greenhouse gases," said Richard Smardon, an environmental scientist and one of the researchers involved.
People have not yet figured out "how to stick the 'urban vegetation' into that equation," he said.
During photosynthesis, trees remove carbon from the atmosphere and store it away in the tree's wood. This process, known as sequestration, reduces levels of carbon dioxide (CO2), a greenhouse gas, in the air. Trees also provide shade and lower air temperatures, reducing the amount of energy that buildings use and, therefore, the amount of work required—and CO2 emitted—by power plants.
Trees with denser wood, such as hawthorn trees, are most effective at removing carbon dioxide from the air than others. Other trees emit what are known as volatile organic compounds, which can contribute to the formation of ozone. Although ozone in the upper levels of Earth's atmosphere has a protective effect, particles of ozone in the air we breathe are considered a pollutant.
To create the ideal combination of trees for Syracuse, the group chose trees with an optimal carbon-sequestering ability and minimal emissions of volatile organic compounds. They also considered practical concerns for urban trees: For example, it is necessary to include a diversity of tree species and best to avoid trees that are very susceptible to disease, such as the American elm. Large and long-lived trees are also crucial, particularly for the shade they provide.
The researchers suggest that the optimal vegetation for Syracuse would be a group of 31 different types of trees, including dogwood, red hickory, and hawthorn. Additionally, the trees would be most valuable if they were planted in the center of the city, where areas of continuous asphalt typically send CO2 straight into the atmosphere.
If the recommended combination of trees were planted, it could reduce CO2 by 2 percent by the year 2046, the team believes.
But the potential impact of an optimal urban forest is far greater in cities that are warmer than Syracuse and have a faster rate of growth. For instance, in Atlanta, trees could make a "tremendous" impact on greenhouse gas levels, Smardon said, potentially reducing carbon dioxide levels by 5 to 10 percent.
Calculated tree selection can be even more valuable in reducing ozone and other particles, which lower air quality, said David Nowak, an ecologist with the U.S. Department of Agriculture Forest Service, who consulted on the study. Because trees lower air temperature, they reduce the formation of ozone, which forms when the air is warm. Additionally, trees that have leaves with rough surfaces can capture the particles that pollute the air.
More and more cities will want to conduct studies similar to the SUNY report, predicts Greg McPherson, director of the USDA's Center for Urban Forest Research.
"At this point, no one is really paying people to plant trees as carbon offsets," said McPherson. "I think urban forestry is going to become a much more viable greenhouse gas reduction measure."
Meryl Rothstein